1. Field of the Invention
The present invention relates to a reduced glare imaging system for motor vehicles, and more particularly to roadway illumination systems which do not cause glare for oncoming drivers.
2. Discussion of the Related Art
Both the human and economic costs resulting from automobile accidents are staggering. In 1994 alone the economic cost of automobile accidents was more than $150.5 billion. By 2000 the annual cost had skyrocketed to $230.6 billion, and the cost continues to rise. Tragically, the toll on human life is even more devastating, especially at night. Nighttime driving represents only about 28 percent of total driving, yet it accounts for about 55 percent of all traffic fatalities. On a per mile basis, driving at night is more than three times as likely to result in a fatality as compared to driving during daylight. In 1996 alone there were more than 18,000 fatal nighttime automobile accidents, including approximately 3,500 pedestrian fatalities and 368 bicyclist fatalities. Significantly, nighttime pedestrian fatalities represent about two-thirds of all pedestrian fatalities caused by automobiles. While several factors affect these statistics, limited vision is one of the main reasons behind the high rate of automobile accidents and fatalities. In particular, a large percentage of nighttime car accidents occur either due to inadequate illumination of the roadway or due to drivers being blinded by oncoming cars.
Several scientific conceptions are currently under different stages of development to improve driving safety at night. One concept uses ultra-violet (UV) light, which is invisible to oncoming drivers, to supplement an automobile's high beam headlights. This method is described in U.S. Pat. No. 4,970,628 to Bergkvist. Another automobile headlight concept using UV light is disclosed in U.S. Pat. No. 5,255,163 to Neumann (Neumann). Neumann discloses a headlight for a motor vehicle which includes a gas discharge lamp as a light source emitting both UV and visible light.
UV road illumination has several substantial drawbacks, however. Notably, UV light does not adequately illuminate many obstacles on the road. Hence, if a driver becomes too reliant on UV lamps, the driver may miss important imaging information, which increases the probability of car accidents at night. Fluorescing materials which improve illumination in the UV spectrum can be installed into roads, but in the U.S. alone the expense of installing the fluorescing materials onto all roadways will run into the billions of dollars. Further, many natural objects will still be difficult to see if only UV illumination is used to illuminate a roadway. Thus, even if UV road illumination is implemented into vehicles, low beam headlights will probably still be used to insure adequate illumination. Low beam headlights, however, can produce glare for oncoming drivers.
UV light also can be hazardous to pedestrians and oncoming drivers since UV light emanating from an automobile's headlights is likely be brighter than ambient UV light received from the sun on a typical summer afternoon. Since pedestrians and oncoming drivers will not see the UV light, they likely will not close their eyes as they would if they were looking directly at the sun. Notably, the eye of a pedestrian is likely to be opened wider during the night as compared to the day. Moreover, the pupil of an eye is one to two orders of magnitude larger at night. Accordingly, pedestrians and oncoming drivers are likely to receive a total exposure of UV light which can be damaging to their eyes.
Infrared (IR) thermal imaging using light having a wavelength of approximately 9-10 μm is another illumination concept currently being developed. In fact, IR thermal imaging cameras are commercially available on certain automobiles. Thermal imaging has several drawbacks, however. Significantly, since the 9-10 μm wavelength is 20 times longer than visible radiation, the spatial resolution of an image generated by IR thermal imaging is 20 times worse than the resolution obtained using visible light. In particular, an IR thermal image has a resolution which is typically only 76800 pixels (320×240), two orders of magnitude less than the resolution of modern charged coupled device (CCD) cameras, or the human eye.
In addition to the resolution limitations of IR thermal imaging, road image contrast, sharpness and brightness of an IR thermal imaging system is dependent on ambient temperature. Objects on a road which have equal temperature, for example tires, trees or stones on the road, might not be distinguishable. For example, if an ambient temperature is close to the temperature of a human body (36° C.), humans will not be seen or will be seen with poorly distinguishable contrast. If the ambient temperature is too cold, for example −25° C., the brightness and the contrast of the IR thermal images might be two to three times worse in comparison to images taken with a warmer ambient temperature, for instance +25° C. Another issue with IR thermal imaging is that an image of an object which is taken during a rain storm, or immediately thereafter, will be different than an image of the same object which is taken when ambient conditions are dry.
A number of other active and semi-active night viewing devices are known. Such systems often use a target illumination system which is pulsed, such as a pulsed laser, and an imaging system. The imaging systems are sometimes gated or provided with a spectrally selective filter in an attempt to filter out visible light from oncoming automobiles. However, current systems using these techniques are not able to block enough visible light from oncoming vehicle headlights to provide high resolution images. For instance, the period between laser pulses is not adequate to provide a precise image. Moreover, spectral filters currently used are not sufficiently selective to distinguish scattered light from light generated by headlights of oncoming vehicles. Further, the energy required for generating the illumination pulses is quite high in order to have an acceptable signal to noise ratio.